I didn't study for it and made an R the first time I wrote. Keep in mind:
1) The essays are scored by one human and one computer. Even though I've seen some people speculate there is advanced reading software used by these computers, I think it's more likely the computer can't really recognize the coherency of an argument and is looking for good grammar and "buzz words." I rehashed the prompt in the first sentence of each of my three paragraphs. Give yourself a little time at the end of the half hour to reread your essay quickly to check for grammar.
2) As another poster stated:
First paragraph: When is the prompt true?
Second paragraph: When isn't the prompt true?
Third paragraph: How can you tell?
3) According to SDN, writing scores are relatively insignificant in the admissions process. I personally think that a good writing score can't really help your application, but a poor writing score (L or lower) can keep you out. Of course, if you can put together halfway decent essays, you're in business. IMHO, the writing section is just there to make you more tired for the BS section.
Good luck.